Devices for measuring weak magnetic fields are used commercially. Such a device is used in equipment for providing monetary change in order to distinguish between bank notes which may have the same size and color. The bank notes are inscribed with a magnetic ink to enable the values to be detected. The bank notes in the monetary change dispenser are subjected to a constant magnetic field which magnetizes the magnetic ink in a predetermined direction before the bank notes are conveyed past the device for measuring the weak magnetic field to be used for identifying the values of the bank notes.
The magnetic ink retains a residual magnetic field after passing through the constant magnetic field so that the characters in the magnetic ink act as tiny permanent magnets oriented in the same spatial direction. Such residual magnetic fields are weak and exhibit a magnetic flux on the order of approximately 2.5 Webers per square millimeter. The device for measuring the weak magnetic field is used to measure the magnetic fields produced by the characters which are acting as permanent magnets.
The inscriptions on the bank notes differ from each other in accordance with the value of the bank note and these differences are detected in the magnetic fields. The device should scan the bank notes without touching them and the distance between the device and the bank note is usually at least one millimeter. This minimal distance is desirable because bank notes can be crumpled and the descriptions on the bank notes should not be worn off or damaged through friction as the bank notes are conveyed past the device.
Devices for measuring low magnetic fields have other applications. For example, electrical meters and power meters can obtain a measurement of electrical current by making a measurement of the magnetic field which is proportional to that electrical current.
U.S. Pat. No. 3,280,974 discloses a device for measuring weak magnetic fields for determining the value of U.S. bank notes, particularly FIGS. 8 and 9 of the patent. The patent discloses a curved fluxgate operating on a well-known principle of the saturation core probe. A conventional, rod-shaped fluxgate is well suited for measuring weak magnetic fields because of its high sensitivity due to a large number of windings and because of its high resolution. The conventional, rod-shaped fluxgate is, however, less suited for this application because its configuration is not suited for plotting weak magnetic field sources having low magnetic fluxes. Such a fluxgate is disclosed in the prior art and includes a magnetic core having a very low magnetic reluctance material such as a plate made of Mu-metal. The present commercial technology presents difficulties in producing two identical fluxgates to compensate for the terrestrial magnetic field using a compensating circuit including two probes.